The present invention relates generally to telecommunication systems, and more particularly to a resistance bridge for interconnecting multiple stations in a 4-wire full conference system.
The stations or terminals in most low frequency telecommunication systems, including most public telephone systems, are interconnected by 2-wire transmission lines. If full duplex operation is desired (as in a telephone system, for example), a hybrid transformer or other suitable means must be provided at each station to enable the single pair of wires simultaneously to carry messages traveling in opposite directions. A certain amount of cross-coupling does occur, but usually it can be held within acceptable limits by balancing the line carefully and avoiding excessive signal levels. When maximum separation between incoming and outgoing signals is required , however, 4-wire transmission lines are used to join the stations in a system.
A 4-wire transmission line consists of two associated pairs of wires. One pair carries information in one direction between two interconnected stations; the other pair carries information in the opposite direction. Such lines are commonly used in multistation private line networks, for example, and in other applications where maximum separation is needed for circuit stability.
The transmission lines in a 4-wire telecommunication system normally are interconnected by means of a 4-wire bridge having an input (receive) and an output (transmit) port for each line. Such bridges, which take the form of passive resistance networks, ideally have an infinite return loss (i.e., zero energy transfer) between the input and output ports of the same line. In addition, an "ideal" bridge has a uniform natural loss between each line's input port and the other lines' output ports, and provides the proper terminating impedance for each of the interconnected transmission lines. Thus, in an ideal bridge signals received at any line's input port will appear (in equally attenuated form) at the output ports of all the other lines, but will be absent at that line's own output port.
Four-wire resistance bridges of the type being considered typically are constructed using discrete resistors, the number required being equal to 2(N.sup.2 -N), where N is the number of lines the bridge is designed to accomodate. Thus, a 4-wire, 3-line (or 3-leg) bridge includes 12 resistors, all of the same value. A schematic diagram for such a bridge is shown in FIG. 2. It will be noted that there is no direct connection between the input and output terminals of the same leg, or line, but that there are multiple secondary paths between them. For example, input terminal RT1 is not connected directly to output terminal TT1, but the two terminals are joined by a secondary path that includes resistors R2, R6 and R5. Another, more remote secondary path between the same terminals includes resistors R1, R12, R11, R7, R8, R4, R3, R10 and R9. To prevent signals received at the input port of a line from being returned to the sender via that line's output port, the bridge must be wired so as to cancel the energy in these secondary paths. This is accomplished by incorporating "rollovers" in certain of the primary paths, the remaining paths being wired "straight." A straight connection is one in which the Receive Tip (RT) and Receive Ring (RR) terminals of one line are connected directly to the Transmit Tip (TT) and Transmit Ring (TR) terminals, respectively, of a different line. In a rollover connection, the RT and RR terminal of a line are cross-connected to the TR and TT terminals, respectively, of a different line. The FIG. 2 bridge includes five straight connections and one roll-over (between line 1 and line 2).
The number of straight-rollover connection combinations in a 4-wire bridge is equal to 2.sup.N(N-1). Thus, the number of possible combinations increases from 32 for a 3-leg bridge to approximately 7.2X10.sup.16 for an 8-leg bridge. For that reason, it has not been considered possible, as a practical matter, to construct an ideal 4-wire, 8-leg bridge, even though correct straight-rollover patterns have been discovered for 3-, 4- and 6-leg bridges. When more than six lines are needed for a telecommunication circuit, standard practice has been to use combinations of 4-leg and 6-leg bridges tied together with amplifiers. A nominal 600 ohm impedance (the standard value for 4-wire telecommunication lines) 4-leg bridge has a natural line-to-line loss of about 14.7 dBm, and a 6-leg bridge has a natural loss of about 19.5 dBm. On standard toll lines, signals are received at +7 dBm and sent out at -16 dBm, a total of 23 dBm loss from input to output. Accordingly, it is necessary to provide additional padding (loss) to match standard toll levels when 4- and 6-leg bridges are used. It can be shown, however, that a 600 ohm 8-leg bridge has a natural loss of about 22.6 dBm. Since 0.5 dBm loss is normally allowed for intra-office cabling, no additional padding is required with a 4-wire, 8-leg bridge. In view of the above, the primary object of the present invention is to provide an ideal 4-wire, 8-leg bridge suitable for interconnecting multiple stations in a 4-wire full conference system.
A more specific object of the invention is to provide a 4-wire, 8-leg resistance bridge having a pattern of straight and rollover connections between the various legs that provides an infinite return loss between the input and output terminals of the same leg, together with a uniform natural loss between the input and output terminals of different legs.
The full, lawful scope of the present invention is set forth with particularity in the appended claims. However, the various objects, features and advantages of the invention will be best understood and appreciated by reference to the following description of the preferred embodiment read in conjunction with the accompanying drawing.